Typically, DC-DC converters use current flow information to provide value added functions and features. For example, limiting the current during an overload is commonly implemented as a safety feature. Such a current limit feature would use a signal proportional to output current limiting level. A resistor inserted between the output and the load could generate the desired signal. However, the resistance of this sensor is the subject of a trade-off between power dissipation and signal amplitude. Typically, the signal level at current limit is approximately 0.1 volt, to be well above the noise floor. The sensing resistor's power dissipation is proportional to the load current at the limit level. At high current levels, the power dissipation can be excessive.
Eliminating the sensing resistor improves the DC-DC converter's efficiency. Instead of an additional resistive element, current flow is measured using the intrinsic elements within the power converter components. For example, U.S. Pat. No. 5,982,160 to Walters et al. and entitled “DC-to-DC converter with inductor current sensing and related methods” teaches that the current flow information in an inductor can be reconstructed as a voltage across a resistor-capacitor network. This method uses the intrinsic resistance of the inductor's winding as the current sensing element.
Another method to eliminate the current sensing resistor measures the voltage dropped across the nearly constant, on-state resistance of one of the switching MOSFETs in the converter. The method samples the voltage drop during the conduction interval of the MOSFET to reconstruct the current flow information. Both of these methods make use of the fundamental power converter components as current sensing elements and they avoid using a dissipative element in the power path.
The intrinsic current sensing methods in the above examples can only approximate the actual current flow. These methods suffer in accuracy when compared with the current sensing resistor. For example, utilizing the inductor's winding resistance as the current sensing element suffers both an initial tolerance error and a variation with temperature. An inductor's winding initial resistance varies with the length and diameter of the winding's wire, as well as the specific manufacturing procedure. This same wire resistance increases as a function of temperature. Therefore, the reconstructed voltage signal is a function of the inductor windings' mechanical tolerance and temperature as well as the current flow.